The present invention reduces effects of magnetic interference that can corrupt magnetic field signals received from an underground transponder associated with an underground boring tool. The magnetic field signals are used to determine the location of the underground transponder.
Several guided and unguided sub-surface (i.e., underground) boring tools are currently on the market. Guided tools require substantially continuous location and orientation monitoring to provide necessary steering information. To monitor such an underground tool it is necessary to track the sub-surface location of the tool. Only once the location of the tool is located can a proper depth measurement be obtained, for example, from a measuring position directly above the head of the boring tool which houses a transmitter. Unguided tools would also benefit from periodic locating or substantially continuous monitoring, for example, in prevention of significant deviation from planned tool pathways and close tool approaches to utilities or other sub-surface obstructions.
One method for locating such sub-surface boring tools includes mounting a magnetic field source on the boring tool and detecting the magnetic field from that field source. This field source can be, for example, a solenoid, or any equivalent transponder capable of generating the magnetic field. When alternating current flows through the solenoid a bipolar magnetic field is thereby generated, which can be detected at the surface by a monitoring device. A vertical component of the magnetic field at the surface will change direction when the monitoring device is directly above the solenoid, assuming the solenoid is horizontal. Therefore by noting the position in which that component of the field reverses, the position of the solenoid in a horizontal plane can be determined. If this is done continuously, the movement of the boring tool on which the solenoid is mounted can be tracked. The depth of the solenoid can also be gauged by measuring the attenuation of the field at the surface. This requires the field strength at the solenoid to be known.
As described above, determinations of the location of underground boring tools rely upon magnetic field measurements. Thus, the reliability and accuracy of such location determinations can be adversely affected when the magnetic field measurements are corrupted. More specifically, the location determinations can be adversely affected at the monitoring device. The primary sources of magnetic field interference in this environment are power distribution networks. Overhead and/or underground power lines of such power distribution networks produce harmonically derived interference signals at regular harmonic intervals of their fundamental frequencies, 50 Hz (xc2x10.1 Hz) or 60 Hz (xc2x10.1 Hz), through to well above 10 kHz. Besides adversely affecting the reliability and accuracy of location determinations, magnetic field interference can cause instability in location determinations calculated by the monitoring device, thereby causing a location display to appear unstable to an observer (i.e., user of the monitoring device). Accordingly, there is a need to reduce the effects of such interference, to thereby improve the reliability and accuracy of location determinations, and the stability of a display of the location.
It is often useful to know more than just the location of a boring tool. For example, it is often useful to know the orientation (e.g., yaw, pitch and/or roll) of the tool. To provide this information, the magnetic field generated (e.g., by the underground transponder) is modulated to impart modulated information thereon that can be demodulated and thus obtained (made available) at the monitoring device. Existing monitoring systems provide limited data throughput (i.e., data transmission bandwidth from the transponder to the monitoring device), in part due to the need to avoid data corruption by magnetic field interference. Accordingly, there is a need to increase the data throughput that can be achieved in an environment that includes the interference described above.
The present invention is directed to methods and systems for reducing effects of magnetic field interference that may interfere with a magnetic field signal generated at or near an underground object, where the magnetic field signal is used to monitor the location of the underground object. An embodiment of the present invention includes producing at least two different moving averages of a plurality of samples that are representative of a detected magnetic field signal strength. Each of the different moving averages is a moving average of a different number of the plurality of samples. A respective quality metric is then determined for each of the different moving average. One of the moving averages is selected based on the determined quality metrics. Further signal processing is then performed using the selected moving average. For example, the selected moving average is used to monitor the location of the object.
In an embodiment of the present invention, each of the moving averages corresponds to a respective moving average length that is representative of the time interval during which the samples within the moving average were produced. Each of the moving average lengths are different from one another. Preferably, the moving average lengths collectively span the range of expected interference cycle periods. Preferably, the moving average lengths collectively span an operating bandwidth of the system that calculates magnetic field strength.
According to an embodiment of the present invention, an additional moving average (having a moving average length equal to the selected moving average) of additional samples representative of the magnetic field signal is produced. Further signal processing is then preformed using the additional moving average. For example, the additional moving average is used to monitor the location of the object.
In an embodiment of the present invention, the quality metric is a measure of variance. That is, a respective variance is determined for each of the different moving averages. Then the one of the moving averages associated with the lowest variance is selected as the preferred moving average.
In another embodiment, the quality metric for each of the moving averages is a difference between a respective maximum and minimum of each moving average. The preferred moving average is the one with the lowest difference. In another embodiment, the preferred moving average is the one with the lowest ratio between its respective maximum and minimum.
The different moving averages can be concurrently produced. Alternatively, the different moving averages are produced serially.
According to an embodiment of the present invention, a preferred moving average length is selected based on a plurality of samples that are representative of a detected magnetic field signal. A moving average of an additional plurality of samples that are representative of the detected magnetic field signal are then produced using the preferred moving average length. Further signal processing is then performed using the moving average produced with the additional plurality of samples. For example, the moving average is used to monitor the location of an underground object (e.g., a boring tool). Each of the different moving average lengths corresponds to a time over which the samples within the moving average span. The different moving average lengths are preferably within a range of expected interference cycle periods. The different moving average lengths should collectively span the range of expected interference cycle periods. If the samples are produced with a constant sample spacing, then each moving average length is defined by a product of the number of samples in the respective moving average and the sample spacing. Preferably, the moving average lengths collectively span an operating bandwidth of the system that calculates magnetic field strength.
An embodiment of the present invention is directed to a subsystem for use in a device that monitors the location of an underground object based on a magnetic field generated at or near the object. The subsystem reduces effects of magnetic field interference that may interfere with the magnetic field generated at or near the object. The subsystem includes a means for producing at least two different moving averages of the plurality of samples that are representative of a detected magnetic field signal. Each of the different moving averages is a moving average of a different number of the plurality of samples. The subsystem also includes a means for determining a respective quality metric for each of the different moving averages, and means for selecting one of the moving averages based on the determined quality metrics. Further signal processing can then be performed (e.g., in a system computer) using the selected moving average. The further signal processing may include monitoring the location of the object.
A subsystem of the present invention includes a selector for selecting a preferred moving average length based on a plurality of samples that are representative of a detected magnetic field signal. The subsystem also includes an averager to produce, using the preferred moving average length, a moving average of an additional plurality of samples that are representative of the detected magnetic field signal. A system computer then performs further signal processing using the moving average produced by the averager. The system computer, for example, uses the moving average to monitor the location of an underground object.